Structural phase transitions of calcium and strontium under high pressure

Abstract

The Group IIA metal, also known as alkaline-earth, undergoes the structural phase transitions under high pressure due to s to d orbital electron transition. It has been reported to possess several novel crystal structures, however, the full structural details has not been identified. In this thesis, structural studies of these metals have been carried out using computational method. Density Functional Theory (DFT) and Molecular Dynamics (MD) are the computation approaches which have been exploited to calculate the transition behavior. For calcium, the ambient crystal structure is the face-centered cubic (fcc) with space group. Under higher pressure, it transforms to the body-centered cubic (bcc) with space group at 5.4 GPa and then transforms to the Ca-III structure which is the β-tin structure with space group at 33.2 GPa. Finally, it transforms to the structure at 91.8 GPa. For strontium, at medium pressure range (20 GPa-40 GPa), structures were investigated by ab initio calculation and special interest has been given to the variation of exchange-correlation functional, i.e., Perdew Burke Ernzerhof (PBE) and screened exhange-Local Density Aproximation (sX-LDA). By using PBE potential functional in the calculation, the fcc structure shows the transformation to the bcc structure at 1.4 GPa and then to the hcp structure at 23.8 GPa. On the contrary, when using sX-LDA as a potential functional, the enthalpy calculation of β-tin structure is significantly lower than those of the hcp structure. Therefore, sX-LDA is the most suitable potential functional for this type of phase transitions as sX-LDA can provide a significantly improved result when compared with the experimental data.